Various methods and systems for producing microwave induced hyperthermia in tumors have been proposed in the prior art.
These methods, in general rely principally on the fact that a tumor, because of a slower rate of blood profusion, will be preferentially heated with respect to adjacent normal tissue when the affected area is placed in a high frequency alternating electromagnetic field.
A tumor will be necrosed when heated to a temperature of between 46.degree. C. and 50.degree. C. for a sufficiently long period of time. Heating to these temperatures however will severely injure normal tissue.
It has been found that in certain cases it is possible to keep the temperature of normal tissue below approximately 40.degree. C., which results in minimal or no injury, while raising tumor tissues to high energy temperature levels to necrose the tumor.
U.S. Pat. No. 2,407,690 to Southworth discloses a microwave system for heating a liquid, or treating living tissue by applying microwave energy from a waveguide through impedance couplers, and various shapes of applicators placed in contact with the patient.
U.S. Pat. Nos. 3,991,770, 4,095,602 and 4,119,102 to LeVeen disclose systems for radio frequency heating of tissue disposed between two metallic plates connected to the output of a radio frequency generator, so that the tissue has induced within it a high frequency alternating electric field.
U.S. Pat. No. 4,121,592 to Whalley discloses microwave tissue heating apparatus utilizing tissue surface conforming metal sponges to prevent localized areas of surface tissue from overheating.
U.S. Pat. No. 3,800,802 to Berry et al. discloses an apparatus having a plurality of radio frequency magnetic induction coil heads to treat more than one patient, or different parts of the same patient simultaneously.
"Physical Hyperthermia and Cancer Therapy", by Short and Turner in Proceedings of the IEEE, Volume 68, No. 1 (January, 1980) provides a general summary of developments in the field of hyperthermia including reference to a system which uses "chilled distilled water . . . circulated through a chamber on the front of contact applicators to draw off excess heat from the skin." (See p. 138).
While these prior art systems seemed promising, precise control of heating of tumor and healthy tissue was difficult to achieve. Often regions of healthy tissue were injured or portions of tumors left unharmed.
The addition efficient use of microwave energy would not occur because a substantial fraction of the microwave energy is subject to reflection when encountering an interface between media of substantially different electromagnetic properties, for example air and tissue, which have substantially different intrinsic impedance. This results in a lack of penetration of the microwave energy.
A further difficulty resulting from mismatch in intrinsic impedance is surface heating of the tissue. Thus skin and underlying tissue structure are subject to severe damage.